CNG has no harmful effects provided your car is well tuned and the proper combustion is occuring in the engine,
if it had any harmful effects then my corolla 1.6 wont have done 1,95,000 km without any prob on CNG although i get the oil changed at 10,000km and car runs mostly on motorway / highway. The Engine oil i use is a ZIC A+ Fully Synthetic VSVI API/SM.
Engine still is in a 100% cond